The present invention relates to a fuel injection system.
In internal combustion engines having spark ignition of a compressed fuel mixture with internal formation of the mixture, a xe2x80x9cmixture cloudxe2x80x9d which must have a certain fuel-air ratio in the ignitable range is required for stratified charge operation in the spark plug area. To this end, fuel injectors having nozzles which open toward the inside or the outside and produce a conical jet are used.
For example, German Published Patent Application No. 198 04 463 describes a fuel injection system for internal combustion engines having spark ignition of a compressed fuel mixture, this fuel injection system is provided with at least one fuel injector which injects fuel into a combustion chamber formed by a piston/cylinder arrangement and is equipped with a spark plug projecting into the combustion chamber. The nozzle body of the fuel injector is provided with at least one row of injection holes distributed over the circumference of a nozzle body of the fuel injector. Through controlled injection of fuel through the injection holes, a jet-guided combustion method is implemented by the formation of a mixture cloud, at least one jet being directed in the direction of the spark plug. Other jets ensure that an at least approximately closed or coherent mixture cloud is formed.
German Patent No. 196 42 653 describes a method of forming an ignitable fuel-air mixture. An ignitable fuel-air mixture is formable in the cylinders of direct-injection internal combustion engines, in that fuel is injected into each combustion chamber delimited by a piston, by way of an injector on opening of a nozzle orifice due to a valve element being lifted up from a valve seat surrounding the nozzle orifice. To permit formation of a mixture optimized for fuel consumption and emissions in each operating point of the entire engine characteristics map under all operating conditions of the internal combustion engine, in particular in stratified charge operation, the opening stroke of the valve element and the injection time are adjustable.
German Patent No. 38 08 635 describes a fuel injection device for direct injection of fuel into the cylinder of an internal combustion engine having compression of a fuel mixture. The fuel injection device includes a fuel injector which is situated in the cylinder wall at a distance from the cylinder head and opposite the exhaust opening and which has an outlet opening, with the axis of the jet of the injection valve being directed at the area around the spark plug situated in the cylinder head. The fuel injector here has a magnetically operated valve needle having helical swirl grooves to produce a swirl flow of the injection jet. The total cross-sectional area of the swirl grooves is smaller by at least one half than the cross-sectional area of the outlet opening, the fuel injector being situated above the flushing opening, and with its jet axis directed at the ignition point situated at the center of the cylinder head.
Most injection systems known from the publications cited above concern combustion methods with wall-guided or air-guided fuel flow. The air-guided combustion method depends to a very great extent on the movement of incoming air, which has the function of conveying an ignitable fuel-air mixture exactly into the electrode area of the spark plug over the entire stratified charge operation range of the engine characteristics map. In the wall-guided combustion method, fuel is carried to the spark plug with the support of more or less fractured combustion chamber geometries with simultaneous formation of the mixture.
Transport of the mixture to the spark plug is very incomplete in wall-guided and air-guided combustion methods in idling operation and in the lower partial load range, and in the middle partial load range of operation it is possible in part only with unjustifiably low manufacturing tolerances of the high-pressure injectors used or the flow guidance through the intake manifold. The inadequate reproducibility is apparent in particular in increased emission of unburned hydrocarbons due to isolated instances of misfiring. These properties result in another serious disadvantage of the two combustion methods mentioned above: the engine cannot be operated unthrottled in the idling and lower partial load ranges because due to the great distance between the fuel injector and the spark plug, smaller injection quantities no longer reach the spark plug in the mixture concentration required for stable combustion. This means that the fuel-air mixture at the spark plug electrodes becomes too lean. However, due to the intake air throttling, the advantage in terms of fuel consumption is reduced in comparison with internal combustion engines having compression of a mixture with spark ignition and intake manifold injection.
The fuel injection system according to the present invention has the advantage over the related art that the mixture in the area of the spark plug is not too rich and not too lean due to the design of the injection holes of the fuel injector there, but instead lean bridges are almost entirely prevented due to an approximately closed conical jet shape. The measure according to the present invention supports the development of a stable flame core.
It is also advantageous that the spark plug becomes less sooty, the thermal shock load is reduced, and the lack of sensitivity to the firing angle is improved with a fixed injection time in the entire engine characteristics map in which stratified charge operation is carried out. Injection and ignition may thus take place simultaneously.
The design of elongated injection holes has the advantage that the flow losses during the flow through the injection holes are reduced, so the operating pressure may be lowered.
Various arrangements of the injection holes and different angles of inclination relative to one another may be selected, depending on the requirements of the fuel injection system.
The elongated design of the injection holes enclosing the spark plug is also especially advantageous because the spark plug or the spark plug electrodes can be mounted at a variable mounting depth within a certain tolerance, which is predetermined by the length of the injection holes.